Plants synthesize many classes of organic chemical compounds ranging from simple structures to complex molecules as part of their normal metabolic processes. These compounds are broadly characterised as: (a) primary metabolites which encompass those substances such as nucleic acids, proteins, lipids and polysaccharides that are the fundamental biologically active chemical units of living plant cells, and (b) secondary metabolites which typically have larger, more complex chemical architectures that incorporate one or more primary metabolites into their structures. Various types of secondary metabolites synthesized by plants are commonly referred to as phytochemicals, and include flavonoids, carotenoids, lignans, polyphenols, terpenes, tannins, sterols, alkaloids, saponins, waxes, fats, sugars and gums. It is known that many phytochemicals can significantly affect human metabolism and health, and therefore, there is considerable interest in extraction of these compounds for their incorporation into food products (e.g., functional foods, flavours), nutritional supplements (e.g., nutriceuticals), and in pharmacological preparations. Also, certain classes of phytochemicals are useful for the production of fragrances and for incorporation into topical preparations.
Phytochemicals typically are not soluble in water under ambient conditions due to their organic nature and the preponderance of non-ionic bonds in their architectures. However, they are readily soluble in various organic solvents such as aliphatic alcohols, hexanes, dioxanes, acids, ethers, methylene chloride, trichloroethylene, acetonitrile and the like. Numerous methods are known for extracting phytochemicals from plant materials, most based on sequential extraction processes incorporating one or more organic solvents in combination with washing steps. Some methods teach the use of alkali or alkaline solvents in combination with said organic solvents for increased extraction efficiency. Starting plant materials are usually physically disrupted by means of grinding, shredding, chopping, pulverizing, compressing, or macerating in order to improve extraction efficiencies. Phytochemical extracts produced by such methods must be further processed to remove all trace of the organic solvents, to remove impurities, and to separate and purify individual phytochemicals. Examples of such methods are disclosed in U.S. Pat. No. 5,705,618 issued on Jan. 6, 1998, U.S. Pat. No. 5,925,401 issued on Jul. 20, 1999, U.S. Pat. No. 6,264,853 issued on Jul. 24, 2001, and WIPO International Publication No. 2004/027074 published on Apr. 1, 2004. While such methods are useful for extraction and purification of small quantities of phytochemicals for research purposes, they are difficult to scale to commercial through-put volumes because of the problems associated with cost-effectively, safety and completely removing and recovering the organic solvents from the extracts and spent plant materials. Furthermore, the types and concentrations of organic solvents must be carefully selected in order to avoid structural changes to the target phytochemicals during extraction that may adversely affect one or more of their desirable physical, chemical and biological properties.
It is known that the physical and chemical properties of water within sealed systems can be manipulated by concurrently controlling the temperature and pressure, whereby the water remains in a liquid state even though its temperature is significantly increased above its atmospheric boiling point of 100° C. In this condition, it is known as “subcritical” or “hot/liquid” water. Subcritical water can be maintained in the liquid form until a temperature of 374° C. and a pressure of 221 bars are reached after which, it becomes supercritical water. The polarity, viscosity, surface tension, and disassociation constant of subcriticial water are significantly lowered compared to water at ambient temperature and pressure conditions, thereby significantly altering its chemical properties to approximate those of organic solvents. Consequently, pressurized low-polarity water under subcritical conditions can easily solubilize organic compounds such as phytochemicals which are normally insoluble in ambient water. For example, U.S. Pat. No. 6,001,256 issued on Dec. 14, 1999 and U.S. Pat. No. 6,352,644 issued on Mar. 5, 2002 each describe equipment and methods for extracting volatile aromatic phytochemicals from plants for use as flavours or fragrances wherein subcritical water is produced and maintained at a selected temperature at or above its ambient boiling point of 100° C. However, these methods provide subcritical water at only one temperature during an extraction process thereby enabling extraction of only one class of organic compound from the multiplicity of classes that may be present in the source material.